Thunderbird the client who would not die
Around 2010, I used Thunderbird but switched when it started being neglected and forecast for closure, so I'm surprised it is still limping along.
When Chrome appeared I stuck with FF, but with every update there were more issues with sites loading, so Mozilla pushed me onto Chrome and then through a range of browsers, I'm now with Vivaldi and as it's now launched on IOS, I can have the same bookmarks on my iPad, Android and Windows devices
Firefox is the upstart you want to clip round the ear, in an attempt to get them to regain and fulfill their lost potential. However I fear it is too far gone for those without fond memories of this browser to even consider it